CoO_x nanoparticles embedded in porous graphite carbon nanofibers derived from electrospun polyacrylonitrile@polypyrrole core–shell nanostructures for high-performance supercapacitors

Abstract

A novel composite nanostructure of C–CoO_x–C with CoO_x nanoparticles embedded in N-containing porous graphite carbon nanofibers (CNF) is successfully prepared via sintering the electrospun polyacrylonitrile–cobalt acetate tetrahydrate nanofibers covered by a polypyrrole (PPy) sheath that were attained from the chemical vapor-phase polymerization of pyrrole monomers using concentrated nitric acid as both the dopant and oxidant for the first time. The unique configuration with a well-defined morphology possesses a large specific surface area and prominent conductivity contributed to by the catalysis of metallic Co and the external PPy-derived carbon envelope, which could facilitate effective electron transfer and rapid ion penetration in C–CoO_x–C, thus improving its electrochemical performance. As expected, when employed as an electrode active material for supercapacitors, the resultant C–CoO_x–C showed a more acceptable specific capacitance, better rate capability and higher cycling stability than individual CNF and CoO_x nanoparticle-decorated CNF without the coating of a N-doped carbon layer from PPy (C–CoO_x).

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Introduction

With the excessive depletion of fossil fuels and ever-increasing environmental contamination, the rational design and construction of advanced nanostructures for supercapacitors (SCs) with high energy and power densities, rapid charge–discharge processes, good cycling stability, favorable safety and a relatively low cost is vitally important for developing clean, efficient and renewable alternative power sources to meet the urgent demands for various applications ranging from transportation, electronic and military industry applications to energy storage.^1–5 It is universally known that carbonaceous nanomaterials, metal oxides, sulfides or hydroxides, and conducting polymers are the frequently-used electrode materials for electric double-layer capacitors (EDLCs) and faradic pseudocapacitors, among which metal oxides, sulfides or hydroxides can not only provide a superior energy density compared to traditional carbon-based materials, but they also possess a longer lifespan than conducting polymers.^6–9 Cobalt oxides (Co₃O₄ and CoO), as promising substitutes for RuO₂ in SCs, have attracted tremendous attention over the past couple of decades due to their appealing theoretical specific capacitance (Co₃O₄: 3560 F g⁻¹, CoO: 4292 F g⁻¹), easy processing, comparative inexpensiveness and exceptional chemical durability.^10,11 However, both Co₃O₄ and CoO suffer from the severe problem of poor intrinsic conductivity, which is detrimental to the fast ion diffusion and effective electron delivery required by high-performance SCs, resulting in the actual capacitance being far inferior to the expected value.^12,13

To overcome the above deficiencies, numerous attempts have been made to incorporate nanostructured cobalt oxides (denoted CoO_x) into conductive substrates, such as carbon, metal and conjugated polymer backbones to improve the properties of CoO_x.^14–16 For instance, a well-aligned CoO@polypyrrole nanowire array and a Co₃O₄/flocculent graphene hybrid were respectively immobilized onto three-dimensional (3D) nickel foam, which indeed exhibited extraordinarily high specific capacitances with assistance from the inherent electrochemical activity of each component, the excellent electronic conductivity of the scaffold, the short ion penetration pathway in the ordered structures and the unique synergy effects.^17,18 Nevertheless, when taking the overall weight of the electrode into consideration, the equated gravimetric capacitance would be very low.¹⁹ Normally, for a whole electrode, the larger the content of active material, the more energy the electrode can reserve, but a limit occurs, and increasing the mass loading to a certain extent usually engenders serious loss of the specific capacitance and rate capability, mainly because of enhanced resistance.²⁰ Therefore, it is still challenging and yet imperative to strike a better balance between the electroactive materials and conductive matrix to promote their electrochemical performance.

On the other hand, compared with commercial graphene, carbon nanotubes (CNT) and carbon cloth (CC) with the fiber diameters usually at the micron level, the electrospun carbon nanofibers (CNF) as the electrode materials and supporting frameworks have some advantages in terms of ion transmission, the fabrication process and cost, which are of great concern for practical applications in SCs.^21,22 However, the specific surface area (SSA) of CNF prepared using electrospinning is generally 10–30 m² g⁻¹, one order of magnitude lower than that of graphene (909 m² g⁻¹) or CNT (272 m² g⁻¹),^22–24 hence limiting their charge–discharge ability at the interface of electrode and electrolyte. More recently, researchers have demonstrated that the in situ combination of CoO_x with CNF can create tunable pores for increasing the SSA of the host material, but often with sacrifice of the conductivity, owning to the exudation and decomposition of metal salts during thermal treatments.^12,25 Another effective approach to improve the capacitive behavior of carbon electrodes is to embellish the surface functionalities of porous carbonaceous materials with heteroatoms like N, B, S and P,²⁶ among which N is a more plausible dopant as it could substantially ameliorate the conductivity, surface wettability, adhesion and polarity of the carbon as a consequence of the conjugation between N lone-pair electrons and graphitic π-bonds as well as the more active sites and defects imparted by the partial substitution of N for C atoms.^27–29 Direct carbonization of N-enriched polymers has been identified as a simple and valid strategy to obtain N-doped carbon with a comparatively high concentration and homogeneous distribution of N, which is able to persist in harsh working conditions.^30,31 Polypyrrole (PPy) is an ideal nitrogen source due to its facile conversion of N atoms in pyrrole rings to graphitic N, high nitrogen content and carbon yield.^32,33 To the best of our knowledge, chemical vapor-phase polymerization (CVPP) is a solvent-free versatile technique to obtain conducting polymer layers with a desired thickness, smooth surface and good conductivity that are uniformly clad on different insulating substrates, in which the polymerization commonly proceeds at the oxidant-wrapped matrix in the monomer vapor.³⁴

In light of the above reasons, herein, we report a novel nanocomposite (denoted as C–CoO_x–C) with CoO_x nanoparticles embedded in N-containing porous graphite CNF via sintering the electrospun polyacrylonitrile (PAN)–cobalt acetate tetrahydrate (Co(Ac)₂·4H₂O) nanofibers covered by a PPy shell that originate from the CVPP of pyrrole monomers using concentrated nitric acid (HNO₃) as both the dopant and oxidant for the first time (Scheme 1). As anticipated, when serving as an electrode material for SCs, the obtained C–CoO_x–C showed a more acceptable specific capacitance, higher rate capability and better cycling stability than pure CNF and CoO_x nanoparticle-decorated CNF without the coating of a N-doped carbon layer from PPy (designated as C–CoO_x), which should be attributed to the porous nanostructure with a large SSA, an appropriate equilibrium of exposed graphene edges and defects, outstanding conductivity induced by the external PPy-derived carbon envelope and optimal synergy effects of the constituents.

Scheme 1 Schematic illustrating the synthetic route for C–CoO_x–C.

Experimental

Synthesis of the materials

For the preparation of C–CoO_x–C, the typical procedure is listed below: first, 1.5 g of PAN powder (M_w = 150 000, Sigma-Aldrich) was dissolved in 13.5 g of N,N-dimethylformamide (DMF, Tianjin Tiantai Refine Chemicals Co., Ltd.) at 90 °C for approximately 4 h, followed by the addition of 0.375 g Co(Ac)₂·4H₂O (Sinopharm Chemical Reagent Co., Ltd.) under magnetic stirring to form a uniform solution. Then, the mixture was loaded into a glass capillary with the tip immobilized about 20 cm away from an aluminum foil collector, which was connected to a high-voltage power supply that was maintained at 18 kV. The electrospinning process lasted for about 4 h. Next, the electrospun pale pink textiles (PAN–Co(Ac)₂) were placed into a vacuum desiccator equipped with 0.5 mL of pyrrole (Aladdin) and 0.5 mL of concentrated HNO₃ (Beijing Chemical Works) respectively in two small bottles. After evacuation for about 3 min, the vacuum desiccator was left in an oven at 50 °C for 4 h to prepare the black PAN–Co(Ac)₂–PPy membranes. Finally, the three fabrics PAN, PAN–Co(Ac)₂ and PAN–Co(Ac)₂–PPy were stabilized in a tube furnace at 260 °C for 2 h under air, carbonized at 1000 °C for another 2 h under an argon atmosphere and transformed into the corresponding products CNF, C–CoO_x and C–CoO_x–C.

Characterization

Field-emission scanning electron microscopy (FESEM, FEI Nova NanoSEM), operated at 15 kV, was employed to observe the morphologies of the as-prepared samples. High-resolution transmission electron microscopy (HRTEM) images, fast Fourier transform (FFT) versions, selected area electron diffraction (SAED) patterns and energy dispersive X-ray (EDX) analysis of C–CoO_x–C were obtained using a FEI Tecnai G2 F20 electron microscope at an acceleration voltage of 200 kV, which was installed with a high-angle annular dark-field (HAADF) detector in the scanning TEM (STEM) system. The crystallographic structures were confirmed using X-ray diffraction (XRD, PANalytical B.V. Empyrean) with Cu Kα radiation. Raman characterization was implemented using Horiba LabRAM HR Evolution apparatus with a 633 nm laser as the excitation source. Fourier-transform infrared (FTIR) spectra were obtained using a Bruker Vector 22 spectrometer. X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB250) was applied to investigate the chemical composition of C–CoO_x–C. Multipoint nitrogen adsorption–desorption isotherms were recorded using a Quantachrome Instrument (Autosorb iQ Station 1) at 77.35 K. The weight percent of carbon in the resultant C–CoO_x and C–CoO_x–C was examined using CHN elemental analysis (elementar, Vario EL cube).

Electrochemical measurements

Electrochemical measurements were primarily conducted using a CHI 660C electrochemical workstation (Shanghai Chenhua instrument Co., Ltd.) and a Land Battery workstation (Wuhan Land Instrument Company, China) in 2 M KOH (Beijing Chemical Works) aqueous electrolyte using a conventional three-electrode system, which was assembled with a sample-modified working electrode, a saturated calomel electrode (SCE, reference electrode) and a piece of platinum foil to act as the counter electrode. The working electrode was fabricated as described in our previous works.³⁵ The active material powders (total mass: 8 mg), polytetrafluoroethylene (PTFE, Aladdin) and acetylene black (commercially available) with a weight ratio of 8 : 1 : 1 were blended together in a few drops of ethanol. Afterwards, this slurry was coated over and pressed onto nickel foam current collectors (∼1 cm²) and dried at 40 °C overnight. A potential window of −1.0–0.4 V was selected for all the cyclic voltammetry (CV) tests at various scan rates of 3, 5, 10, 20, 50, 100 and 200 mV s⁻¹, and the galvanostatic charge–discharge (GCD) experiments at multiple current densities of 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 10.0 A g⁻¹ unless otherwise stated. The electrochemical impedance spectroscopy (EIS) spectra were obtained over a frequency range from 10⁵ to 0.01 Hz with an alternating current (AC) amplitude of 5 mV.

Results and discussion

Representative scanning electron microscopy (SEM) images of the electrospun precursor fibers and the annealed products are displayed in Fig. 1 to affirm their morphologies, from which we clearly discovered that the solid CNF (Fig. 1B) were uniform in diameter (150–270 nm) and randomly oriented. In comparison with the pristine PAN (Fig. 1A), the continuous PAN–Co(Ac)₂ microfibers (Fig. 1C) are oblate in cross section with some visible “beads” inside the belts caused by the hydrolysis of Co(Ac)₂. Whereas after the carbonization under an inert atmosphere, some of the resulting C–CoO_x nanostructures (Fig. 1D) had been fractured by larger CoO_x particles (average size: 159 nm, inset in Fig. 1D) situated in or on the straight and round CNF, the diameter of which varied from 230 to 400 nm. A similar spatial distribution of CoO_x particles (average size: 129 nm, inset in Fig. 1F) was also observed for C–CoO_x–C (Fig. 1F) with a rough and porous surface. However, it was not the distinct core–shell architecture but the ribbon-like shape that was retained after sintering treatments in contrast to PAN–Co(Ac)₂–PPy (Fig. 1E), leading to a wider diameter range of 350–700 nm in the confined field of view.

Fig. 1 SEM images of (A) PAN nanofibers, (B) CNF, (C) PAN–Co(Ac)₂ microfibers, (D) C–CoO_x, (E) PAN–Co(Ac)₂–PPy composite and (F) C–CoO_x–C. Insets: size distributions of the CoO_x nanoparticles in (D) C–CoO_x and (F) C–CoO_x–C.

As illustrated in the representative transmission electron microscopy (TEM) image of C–CoO_x–C (Fig. 2A), the majority of the CoO_x granules were encapsulated by graphene lamellae, which could not merely firmly hold the nanoparticles, but also notably improve the conductivity of the local amorphous carbon.²⁵ It was measured and calculated from the high-resolution TEM (HRTEM) images of a naked CoO_x particle at the transversal section (Fig. 2B) and of the graphene layers (Fig. 2C) that the spacings between two adjacent lattice fringes were 0.24 and 0.34 nm, belonging to the characteristic d₃₁₁ of cubic Co₃O₄ phase and d₀₀₂ of graphite carbon, respectively. The ordered lattice shown in the annotated selected area electron diffraction (SAED) pattern (Fig. 2D) of C–CoO_x–C indicated that the bare Co₃O₄ was of monocrystalline nature. Fig. 2E shows the relevant energy dispersive X-ray (EDX) spectrum, where the signals of Cu and Si should be attributed to the carbon-coated copper grid and the instrument substrate, revealing the presence of C, N, O and Co elements in the specimen C–CoO_x–C. The elemental mapping in Fig. 2F shows potent evidence that N atoms were evenly scattered throughout the whole composite nanofiber, while the O element was principally concentrated at the same point as Co.

Fig. 2 Characterizations for C–CoO_x–C: TEM images at (A) low, and (B and C) high magnifications. Inset: the associated FFT version. (D) SAED pattern, (E) EDX spectrum, and (F) HAADF-STEM analysis and EDX mapping of C, N, O and Co elements in the white box area.

To validate the crystallographic features of the sintered samples, X-ray diffraction (XRD) patterns are depicted in Fig. 3A. With regard to the obtained CNF, two broad peaks centered at 24.5 and 43.5° were distinctly detected, which are in complete agreement with the (002) and (100) planes Bragg reflection of graphite. In the case of C–CoO_x and C–CoO_x–C, the just-mentioned peak of (002) facet had narrowed and shifted to a larger diffraction angle (2 theta) of 25.8°, proving that a higher graphitization degree and more exposed graphene edges existed in the carbon matrices of the two products on account of the catalytic and reduction effect of metallic Co.^30,36 Besides, all the other peaks could be lightly assigned to the cubic phase of Co₃O₄ (JCPDS card no. 43-1003) and CoO (JCPDS card no. 48-1719) without any impurities. The Raman spectra of the pure CNF (Fig. 3B) clearly exhibited a pair of peaks at 1345 and 1580 cm⁻¹, corresponding to a defect- or disorder-evoked D band and a crystalline graphitic G band, respectively. As for C–CoO_x and C–CoO_x–C, analogous patterns except for an increased intensity ratio between the D and G bands (I_D/I_G) of the latter were observed with the typical vibration modes of Co₃O₄ and CoO at 190, 466, 508, 606 and 673 cm⁻¹,^16,37 which might be chiefly ascribed to the involvement of N heteroatoms having almost no influence upon the species of CoO_x. It should be noted that much narrower and sharper D and G bands with relatively low I_D/I_G values and a new strong 2D or G′ peak (2680 cm⁻¹) appeared for both C–CoO_x and C–CoO_x–C compared with the synthesized CNF, implying the generation of undisturbed graphitic domains in a larger size due to the metal catalysis in the course of calcination that are absolutely consistent with the XRD data.^38,39

Fig. 3 (A) XRD patterns and (B) Raman spectra of CNF, C–CoO_x and C–CoO_x–C.

The Fourier-transform infrared (FTIR) spectra of the specimens before and after the annealing process were supplemented in Fig. 4 to study their molecular structures. With respect to PAN–Co(Ac)₂ (curve b in Fig. 4A), two peculiar absorption peaks of Co(Ac)₂·4H₂O emerged at 1562 and 1417 cm⁻¹ in contrast to the FTIR spectrum of PAN (curve a in Fig. 4A). As predicted, concentrated HNO₃ could oxidize and polymerize pyrrole monomers in a vapor-phase reaction. In detail, the overlapping band in the region of 3000–3700 cm⁻¹ (curve c in Fig. 4A) is caused by the N–H and unsaturated C–H stretching modes of PPy or the hydroxyl groups (O–H) of adventitious water. Moreover, the FTIR spectrum of PAN–Co(Ac)₂–PPy also shows the existence of fundamental vibrations of pyrrole rings at 1575 (ν_C=C) and 1452 cm⁻¹ (ν_C–C), C–N stretching mode at 1315 cm⁻¹, C–H deformation and wagging vibrations at 1047 and 796 cm⁻¹ as well as a doped state of PPy at 942 cm⁻¹.^40,41 For the prepared CNF-based products (Fig. 4B), the apparent peak located at 1565 cm⁻¹ was verified as a C=C vibration of graphitic domains, and the weak bulge at around 1220 cm⁻¹ could result from the stretching mode of epoxy C–O–C groups. Meanwhile, double bands at 665 and 576 cm⁻¹ related to Co–O bonds in spinel Co₃O₄ and CoO were detected for C–CoO_x and C–CoO_x–C, suggesting the successful implantation of CoO_x in or on the carbon frameworks.^42,43

Fig. 4 FTIR spectra of (a) PAN, (b) PAN–Co(Ac)₂ and (c) PAN–Co(Ac)₂–PPy in (A), and (a) CNF, (b) C–CoO_x and (c) C–CoO_x–C in (B).

Further information on the chemical composition and surface electronic state of C–CoO_x–C was attained from X-ray photoelectron spectroscopy (XPS) measurements. As displayed in Fig. 5A, the three fitted peaks in the C 1s region could be indexed to C–C/C=C (284.7 eV), C–O/C–N (286.2 eV) and C=O (288.6 eV).^44,45 The fine-scan N 1s spectrum in Fig. 5B was deconvoluted into three binding peaks pertaining to pyridinic N (398.6 eV), pyrrolic N (400.2 eV) and graphitic N (401.1 eV), which were responsible for the active sites, unambiguously testifying the retention of N atoms in the graphite carbon structure.^14,46 The high-resolution XPS spectrum of the O 1s region (Fig. 5C) clearly presents the signals of lattice oxygen in CoO_x (530.0 eV), oxygen species in C–OH (532.0 eV) and surface-adsorbed water (533.6 eV).^47,48 In addition, we could effortlessly see from Fig. 5D that the dual predominant peaks of Co 2p_3/2 and Co 2p_1/2 accompanied by two small shake-up satellite lines at 786.2 and 805.3 eV that mostly result from the CoO phase, were immaculately resolved into two spin–orbit doublets positioned at 780.0, 795.1 eV and 781.6, 796.6 eV, separately associated with Co³⁺ and Co²⁺ in spinel Co₃O₄,^49,50 attesting to the fact that the CoO_x nanoparticles exposed on the surface of C–CoO_x–C were prevailingly in the form of Co₃O₄.

Fig. 5 XPS spectra of C–CoO_x–C: (A) C 1s, (B) N 1s, (C) O 1s and (D) Co 2p regions.

The SSA and pore parameters of C–CoO_x–C were estimated using the multipoint nitrogen adsorption–desorption isotherms (Fig. 6) collected after degassing treatment at 150 °C, which resembled the type IV isotherms with characteristic H3 hysteresis loops (IUPAC classification), signifying the formation of mesoporous structures and slit-shaped pores. It is worth emphasizing that the adsorption capacity displayed a sudden rise at high pressures (0.9 < P/P₀ < 1) without a plateau, thus hinting at the existence of macropores in the sample.⁵¹ The Brunauer–Emmett–Teller (BET) surface area of C–CoO_x–C was estimated to be 207.7 m² g⁻¹, much larger than that of the reported electrospun CNF,²² allowing a sufficient contact of active materials with electrolyte. As could be concluded from the homologous pore size distribution (PSD) curve (inset in Fig. 6), the most probable Barrett–Joyner–Halenda (BJH) desorption pore diameter is 3.82 nm. The mesopores together with macropores were supposed to facilitate mass transfer, and then elevate the electrochemical performance by offering passageways for ions and accommodation for charges.^3,35,52

Fig. 6 Nitrogen adsorption–desorption isotherms of C–CoO_x–C. Inset: the PSD curve originating from the desorption branch.

In order to elucidate the crucial factor playing an important role in the improvement of the electrode properties, electrochemical impedance spectra (EIS) (exhibited in Fig. 7A) were fitted using an equivalent circuit (inset in Fig. 7A) composed of R_s, the intrinsic resistance of the active materials and bulk solution, R_ct, the charge-transfer resistance, W, the Warburg impedance, and C₁ and C₂, the double-layer capacitance and pseudocapacitance. In the low frequency area, the tail sections of the plots for the C–CoO_x and C–CoO_x–C electrodes inclined nearly with the same slope to the imaginary axis, which were more vertical than that of the CNF electrode, demonstrating that the specimens including CoO_x particles possessed an equally smaller diffusion resistance. The R_ct of C–CoO_x–C calculated from the semicircle diameter at high frequency was lower than that of C–CoO_x, indicating that the N-doped carbon modification stemming from the PPy coating could further promote the conductivity and electroactivity of the array. Galvanostatic discharge profiles (Fig. 7B) at a current density of 1.0 A g⁻¹ were used to roughly evaluate the capacitive performance of the obtained electrodes. The specific capacitance was determined in accordance with the following formula: C (F g⁻¹) = (I × Δt) ÷ (m × ΔV), where I (A) is the applied current, Δt (s) is the discharge time, m (g) is the total weight of electrode materials and ΔV (V) is the potential window. In comparison with C–CoO_x (74.8 F g⁻¹), C–CoO_x–C exerted a larger specific capacitance of 107.3 F g⁻¹, commensurate with that of the pristine CNF (100.1 F g⁻¹) thanks to the more effective electron transfer and rapid ion penetration in C–CoO_x–C, mainly due to the CoO_x-induced porous graphite nanostructure, the PPy-derived carbon envelope, the higher carbon content (87.75%) than that of C–CoO_x (77.93%) and the optimal synergy effect.

Fig. 7 (A) Nyquist plots recorded at an open-circuit voltage in the frequency range of 10⁵ to 0.01 Hz. Insets: magnified plots at high frequency and the equivalent circuit diagram. (B) Galvanostatic discharge profiles at a current density of 1.0 A g⁻¹ for CNF, C–CoO_x and C–CoO_x–C electrodes.

Taking the C–CoO_x–C electrode as an example, it was conspicuous that a quasi-rectangular enclosure at negative potential coupled with a pair of redox responses at positive potential that were attributed to the possible reversible conversion between Co²⁺ and Co³⁺ (CoO + OH⁻ ↔ CoOOH + e⁻, Co₃O₄ + OH⁻ + H₂O ↔ 3CoOOH + e⁻) was present in the cyclic voltammetry (CV) curves (Fig. 8A), certifying its double-layer and Faradaic capacitance features.^53–55 With the increase of scan rate, there was an upward trend in the peak current and a minor shift of the peak position caused by the weak electrode polarization.⁵⁶ What’s more, the shape of the CV plots still remained constant even at a higher scan rate of 200 mV s⁻¹, corroborating the good rate capability of the C–CoO_x–C electrode. Fig. 8B shows the current density-dependent galvanostatic charge–discharge (GCD) profiles, the plateaus of which match well with the redox peaks in the CV curves. It was realised that the specific capacitance of C–CoO_x–C gradually declined from 125.7 to 70.6 F g⁻¹ as the current density changed from 0.5 to 10.0 A g⁻¹, revealing a better rate capability of 56.2% than that of the CNF (30.1%) and C–CoO_x (43.6%) electrodes as illustrated in Fig. 8C. According to the literature, the attenuation of the capacitance at extremely high current density might be for the reason that only the outer active sites or pores of the electrode were accessible to electrolyte ions in this circumstance.^57,58 Long-term cycling life is another significant aspect in assessing the electrochemical performance of active materials. The specific capacitance values attained from the discharge branches of the GCD curves at 1.0 A g⁻¹ displayed a sluggish decay during the cyclic test partly owing to the mechanical stress introduced by the insertion and deinsertion of ions.⁵⁹ About 86.0% of the initial capacitance was retained for C–CoO_x–C after 1500 cycles, a little higher than that for C–CoO_x (75.2%). It is believed that the integration of a smaller charge-transfer resistance and faster ion diffusion was conducive to enhancing the capacitive properties of the C–CoO_x–C electrode.

Fig. 8 (A) CV curves at different scan rates and (B) GCD outlines at diverse current densities for the C–CoO_x–C electrode. (C) Specific capacitance and capacitance retention as a function of the discharge current density. (D) Cycling stability of C–CoO_x and C–CoO_x–C electrodes measured at 1.0 A g⁻¹.

Conclusions

In summary, a novel composite nanostructure with CoO_x nanoparticles embedded in N-doped porous graphite CNF has been successfully prepared via calcining the electrospun PAN–Co(Ac)₂–PPy core–shell precursors, the homogeneous and smooth PPy sheath of which was obtained from the CVPP of pyrrole monomers using concentrated HNO₃ as both the dopant and oxidant. When utilized as an electrode material for SCs, the resulting C–CoO_x–C showed a more acceptable specific capacitance of 125.7 F g⁻¹ at 0.5 A g⁻¹, a better rate capability of 56.2% at 10.0 A g⁻¹ and a higher capacitance retention of 86.0% after 1500 cycles than the bare CNF and C–CoO_x, highlighting the advantages of PPy-derived carbon coatings. We believe that this affordable method to construct PPy layers on the desired substrate can be extended to the design of other hybrid architectures for electronics, catalysis, absorbents, sensors and energy storage devices.

Acknowledgements

This work was financially supported by research grants from the National Natural Science Foundation of China (51273075, 21274052, 51473065 and 21474043), and the Graduate Innovation Fund of Jilin University (2015011).

Notes and references

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